linux-kernel - Re: [PATCH 0/4] arm64: Support the TSO memory model

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Message-ID: <f6484dcd-ebf6-4b6f-be17-69b05539e33b@marcan.st>
Date: Fri, 12 Apr 2024 03:43:36 +0900
From: Hector Martin <marcan@...can.st>
To: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 Mark Rutland <mark.rutland@....com>, Zayd Qumsieh <zayd_qumsieh@...le.com>,
 Justin Lu <ih_justin@...le.com>, Ryan Houdek <Houdek.Ryan@...-emu.org>,
 Mark Brown <broonie@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
 Mateusz Guzik <mjguzik@...il.com>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Oliver Upton <oliver.upton@...ux.dev>, Miguel Luis <miguel.luis@...cle.com>,
 Joey Gouly <joey.gouly@....com>, Christoph Paasch <cpaasch@...le.com>,
 Kees Cook <keescook@...omium.org>, Sami Tolvanen <samitolvanen@...gle.com>,
 Baoquan He <bhe@...hat.com>, Joel Granados <j.granados@...sung.com>,
 Dawei Li <dawei.li@...ngroup.cn>, Andrew Morton <akpm@...ux-foundation.org>,
 Florent Revest <revest@...omium.org>, David Hildenbrand <david@...hat.com>,
 Stefan Roesch <shr@...kernel.io>, Andy Chiu <andy.chiu@...ive.com>,
 Josh Triplett <josh@...htriplett.org>, Oleg Nesterov <oleg@...hat.com>,
 Helge Deller <deller@....de>, Zev Weiss <zev@...ilderbeest.net>,
 Ondrej Mosnacek <omosnace@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Asahi Linux <asahi@...ts.linux.dev>
Subject: Re: [PATCH 0/4] arm64: Support the TSO memory model

On 2024/04/11 23:19, Hector Martin wrote:
>>
>> An alternative option is to go down the SPARC RMO route and just enable
>> TSO statically (although presumably in the firmware) for Apple silicon.
>> I'm assuming that has a performance impact for native code?
> 
> Correct. We already have this as a bootloader option, but it is not
> desirable. Plus, userspace code still needs a way to *discover* that TSO
> is enabled for correctness, so it can automatically decide whether to
> use stronger or weaker instructions.

To add some numbers to this (I was just made aware of this paper):

https://www.sra.uni-hannover.de/Publications/2023/tosting-arcs23/wrenger_23_arcs.pdf

Using TSO globally has, on average, a 9% performance hit, so that is
clearly off the table as a general solution.

Meanwhile, more detailed microbenchmarks often show TSO as having better
performance than outright using acquire/release instructions without
TSO. Therefore, just giving up on TSO and using acq/rel semantics for
emulators is also not an acceptable solution.

Additionally, the general load/store instructions on ARM have more
flexible addressing modes than the synchronizing ones, and since general
x86 emulation requires *all* loads and stores to be like this in a
non-TSO model (without much more complex/expensive program analysis to
determine where this can be elided), the perf impact is definitely worse
for emulation (e.g. stack accesses are affected) than for a
microbenchmark where only the "target" test instructions are being modified.

- Hector